HiI have two tables: one is actual customers table and other is staging table

The customers table has a RowID field and when i import data from Customer_Staging i would like to check the max(rowid) and start from the next number(max(rowID)+1)so on ..if the table is null it should start at 1 and then so on......

DECLARE @offset INT
select @offset=isnull(max(RowID),0) from Customers
MERGE Customers AS C
USING (SELECT ROW_NUMBER() OVER ORDER BY ID) AS Seq,*
FROM NewCustomers) AS NC
ON C.LastName = NC. LastName
WHEN MATCHED THEN
UPDATE SET C.FirstName = NC.FirstName
WHEN NOT MATCHED THEN
INSERT (LastName, FirstName,RowID) VALUES (NC.LastName,NC.FirstName,NC.Seq +@offset);